As tensions with Russia simmer, thousands of NATO troops, several warships and dozens of aircraft are taking part in military exercises stretching across the Atlantic, through Europe and into the Black Sea region.
The war games, dubbed Steadfast Defender 21, are aimed at simulating the 30-nation military organisation's response to an attack on any one of its members.
It will test NATO's ability to deploy troops from America and keep supply lines open.
Already in recent years, the US and its allies have deployed troops and equipment in Estonia, Latvia, Lithuania and Poland to try to reassure those members neighbouring Russia that their partners will ride to the rescue should they come under attack.
Russia's decision last month to send thousands of troops to the border area with Ukraine has raised concern at the military alliance, which launched one of its biggest ever defence spending initiatives after Russian troops annexed Ukraine's Crimean Peninsula in 2014.
Top NATO brass insist that the military exercises, involving some 9000 troops from 20 nations, are not aimed at Russia specifically, but they focus on the Black Sea region, where Russia stands accused of blocking the free navigation of ships.
NATO Secretary-General Jens Stoltenberg says the exercises send an important message to any potential adversary: "NATO is ready."
"NATO is there to defend all our allies, and this exercise sends a message about our ability to transport a large number of troops, equipment across the Atlantic, across Europe and also to project maritime power," Stoltenberg told The Associated Press aboard a British aircraft carrier off the coast of Portugal.
The ship, the HMS Queen Elizabeth, is the pride of the British Navy. It's making its maiden voyage and carrying 18 F-35 jets: the first ever deployment of so many of the 5th generation planes aboard an aircraft carrier.
The ship's presence, part of a 6-7 month deployment that will take it south past India, through Southeast Asia to the Philippines Sea, is aimed in part at restoring Britain's tarnished image as a major global power since it left the European Union.
Adorned with high-tech US jets and flanked by warships from other NATO countries, the carrier strike force also stands as an important symbol of unity as the world's biggest security organisation tries to recover from four tumultuous years under the Trump administration.
Stoltenberg will chair a NATO summit in Brussels on June 14 with current US President Joe Biden and his counterparts keen to usher in a new era of trans-Atlantic cooperation, as troops leave its longest-ever mission in Afghanistan while tensions with Russia, and increasingly China, mount.
The war games tie in two new NATO command centres, one in Norfolk, Virginia; the other in Ulm, Germany. Part of the focus of its first phase was to protect the undersea cables that carry masses of commercial and communications data between the US and Europe.
NATO says Russia is mapping the cables' routing and might have darker intentions.
NATO says its policy toward Russia is based on two pillars: strong military deterrence and dialogue. But high-level meetings between the two historic foes are rare, and European officials insist that President Vladimir Putin is turning increasingly authoritarian and distancing himself from the West.
"We're ready to sit down with Russia, because we think it's important to talk, especially when times are difficult," Stoltenberg said.
"The main challenge now is that Russia has not responded positively to our invitation, or our initiative, for a meeting of the NATO-Russia Council," their top consultative forum.